A common technique for error detection of non-real time services is based on Automatic Repeat request (ARQ) schemes which are combined with Forward Error Correction (FEC), called hybrid ARQ. If an error is detected by Cyclic Redundancy Check (CRC), the receiver requests the transmitter to send additional bits of data.
From different existing schemes, the selective-repeat continuous ARQ is most often used in mobile communications. This scheme in connection with FEC will be used for next generation mobile communication systems such as UMTS (Universal Mobile Telecommunications System). A retransmission unit of the RLC (Radio Link Control) layer is referred to as PDU (Protocol Data Unit).
In the art, three different types of ARQ are commonly defined as specified below. Examples of corresponding prior art documents are:    Performance of punctured channel codes with ARQ for multimedia transmission in Rayleigh fading channels; Lou, H. and Cheung, A. S.; 46th. IEEE Vehicle Technology Conference, 1996;    Analysis of a type II hybrid ARQ scheme with code combining, S. Kallel, IEEE Transactions on Communications, Vol. 38#8, August 1990; and    Throughput performance of Memory ARQ schemes, S. Kallel, R. Link, S. Bakhtiyari, IEEE Transactions on Vehicular Technology, Vol. 48#3, May 1999.
Type I: The erroneous PDUs are discarded and a new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU.
Type II: The erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDUs sometimes have higher coding rates and are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.
Type III: Is the same as Type II with the only difference being that every retransmitted PDU is now self-decodable. This implies that the PDU is decodable without the need of forming a combination with previous PDUs. This is useful if some PDUs are so heavily damaged that almost no information is reusable.
Schemes of type II and III are obviously more intelligent and show some performance gain because they have the ability to adjust the coding rate to changing radio environments and to reuse the redundancy of previously transmitted PDUs.
To support incremental redundancy, the sequence number (SN) of the transmission unit has to be encoded separately. The stored data with the known SN can then be combined with subsequent retransmissions.
In the prior art, the SN is encoded in the PDU header or in the time slot header (e.g. EP-A-0938207) and transmitted together with the PDU. If the PDU is corrupted, it is likely that the header is also destroyed. Therefore, the coding has to be done with a lower coding rate to allow the SN to be read even when the data is erroneous. As a result, a large coding overhead exists to ensure reliable transmission of the sequence number. The coding for the SN therefore has to be different from that used for the PDUs resulting into increased complexity. To ensure that the SN is correct, a CRC parity check could be applied, but reliable CRC over a few bits is not very efficient.
Besides the signalling overhead that is introduced by the prior art methods, it is the implementation complexity that has prevented this technique from being used. A large amount of memory is required in the receiver to store the erroneous packets for combining with the retransmissions. Since the SNs are not known before receiving the retransmission, it is not possible to start the combining process before the SNs have been decoded.